Layer sequence for Gunn diode

ABSTRACT

The invention relates to a layered construction for a Gunn diode. The layered construction comprises a series of stacked layers consisting of a first highly doped n d  GaAs layer ( 3 ), a graded AlGaAs layer ( 5 ), which is placed upon the first highly doped layer ( 3 ), whereby the aluminum concentration of this layer, starting from the boundary surface to the first n d  GaAs layer ( 3 ), decreases toward the opposite boundary surface of the AlGaAs layer ( 5 ), and of a second highly doped n +  layer ( 7 ). An undoped intermediate layer ( 4, 6 ) serving as a diffusion or segregation stop layer is placed on at least one boundary surface of the AlGaAs layer ( 5 ) to one of the highly doped layers ( 3, 7 ) and prevents an unwanted doping of the graded layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of PCT applicationPCT/DE2003/003867 filed 21 Nov. 2003 with a claim to the priority ofGerman patent application 10261238.2 itself filed 20 Dec. 2002.

FIELD OF THE INVENTION

The invention relates to a layered structure or layer sequence,especially a layer sequence for a Gunn diode or a Gunn oscillator.

The Gunn effect relates to the appearance, upon the application of aconstant relatively high electrical field strength (electrical fieldstrength above 2000 V/cm) to an n-doped gallium arsenide crystal, ofrapid current fluctuations. It has been found that, with very shortcrystals, these current fluctuations are transformed into relatedoscillations whose frequencies are determined by the length of thecrystal and lie in the microwave range. Also in other semiconductiveIII-V semiconductors, like for example n-doped indium phosphide, amicrowave effect can arise when the electrical field strength exceeds acritical value lying at several thousand V/cm. The effect arisesgenerally in III/V semiconductors whose energy bands have relativemaxima and minima which are spaced by a distance which is not very greatenergetically so that hot electrons can pass easily into higher-lyingbands. If they there have a smaller mobility so that a smaller currentamplitude is associated with the higher field strength, that is anegative resistance is present, the conditions for oscillationgeneration and amplification are present. The Gunn effect is thus usedin so-called Gunn oscillators to produce the microwaves.

From a certain field strength, at an electron emitter, so-calledspontaneous dipole domains can develop which travel through thesemiconductor with a certain speed and end at a collector. When thedomains arrive at the collector, a further domain is formed at theemitter. The emitter thus has a decisive significance in the formationof the domains. At the collector, the charge carriers are collected.

A Gunn diode can typically have an emitter with an AlGaAs layer orientedtoward the collector and which serves to provide a potential drop forthe emitter electrons and thus to boost them energetically relative tothe active diode layer.

Form the publication Greenwald et al. (Greenwald, Z. Woodard, D. W.,Calawa, A. R., Eastman, L. F. (1988) The effect of a high energyinjection on the performance of millimeter wave Gunn oscillators;Solid-State Electronics 31, 1211-1214) a Gunn diode is known which has alayer structure or sequence with an AlGaAs layer of a constant aluminumcontent of 23%.

From U.S. Pat. No. 4,801,982 and Hutchinson et al. (Hutchinson, S., J.Stephens, M. Carr and M. J. Kelly. Implant isolation scheme for currentconfinement in graded-gap Gunn diodes, IEEE Electronics Letters, 32(9),851-852, 1996) it is known to provide an AlGaAs layer with an aluminumcontent which is variable, that is increases in the direction of thecollector linearly from zero to 30%. Such an AlGaAs layer is referred toin English as a graded layer and will be indicated as a graded layerhereinafter. The graded layer can be configured with a steplesstransition to an aluminum content of higher concentration. As a result,advantageously, undesired electron reflections are minimized andelectrons with energies matched to energy bands with low electronmobility can be injected.

The known layer sequence or structure of the state of the art has,because of high fabrication temperatures and/or high temperatures of useof the Gunn diode, the disadvantageous property that the desiredpotential barrier of the AlGaAs layer may be ineffective, that iselectrically inactive. This is especially clear at low input voltagessince then already a linear increase in the current as a function of thevoltage arises.

OBJECT OF THE INVENTION

The object of the invention is thus to provide a layer sequence orstructure which can provide a current/voltage characteristic-line orgraph]—without the disadvantages described as the state of the art.

SUMMARY OF THE INVENTION

The object is achieved by the layer sequence or structure comprised of asequence of layers applied one upon another and with a first highlydoped n_(d)-GaAs layer, a graded AlGaAs layer disposed on the firsthighly doped n_(d)-GaAs layer whereby the aluminum concentration of thislayer decreases from the interface to the first highly doped n_(d)-Ga-Aslayer in the direction of the opposite boundary layer of the AlGaAslayer, and a second highly doped n⁺ layer. On at least one boundarysurface of the AlGaAs layer, juxtaposed to one of the highly dopedlayers, an undoped intermediate layer is arranged.

The AlGaAs layer and the highly doped n⁺ layer form functionally theemitter of a Gunn diode or a Gunn oscillator. The highly doped n_(d)GaAs layer bounding on the emitter serves to finely define the energylevel of the dipole domain which is produced in the direction of acollector.

The undoped intermediate layer is comprised preferably also of GaAs.Such an intermediate layer acts advantageously to impart to thecurrent/voltage characteristic a defined nonlinear relationship, that isa Schottky type of behavior at low voltages.

The starting material of the n⁺ doped electron-emitting layer isadvantageously also GaAs.

In a further feature of the invention, the AlGaAs layer has on both ofits boundary surfaces, the highly doped layers, that is both thebounding n⁺ doped electron-emitting layer as well as the highly dopedn_(d)-GaAs layer, with such an undoped intermediate layer of GaAs beinginterposed between the respective doped layer and the AlGaAs. In thismanner a further improvement of the current/voltage characteristic isobtained in such manner that these intermediate layers both at theemitter side as well as from the collector side of the graded layer willhave a defined electronic structure.

Such a layer sequence or structure can be used as the starting layeredbody for Gunn diodes and Gunn oscillators. In that case, further layersmay be joined to the highly doped n_(d)-GaAs layer for such electroniccomponents.

On the end of the highly doped n_(d)-GaAs layer opposite theelectronic-emitting layer, such further layers can be applied for theseelectronic components including especially a low-doped n⁻-GaAs layer aswill promote the Gunn effect as well as a further highly doped n⁺-layeras a collector-forming layer. The collector layer can especially also becomposed of GaAs.

As the doping agent for the layers, especially silicon and alsotellurium have been found to be advanatageoud. These elements have theeffect of advantageously contributing pure electron conductivity.

The method of making a layer sequence or structure according to theinvention comprises the step whereby upon a highly doped n_(d)-GaAslayer, an undoped GaAs layer is applied and at an appropriatetemperature is epitaxied. On this undoped intermediate layer, an AlGaAslayer is arranged with a variable aluminum concentration. Theconcentration is reduced steplessly away from the GaAs layer at itshighest value, for example 30% to zero %. On the AlGaAs layer, then afurther undoped GaAs layer is applied or a highly doped n⁺-layer can bedirectly applied. The second undoped GaAs layer is then optional.

However, exactly the opposite method sequence can be carried outbeginning with a highly doped n⁺ layer of GaAs.

Thus at least one undoped GaAs layer is applied on at least one of theboundary surfaces of the AlGaAs layer to one or both of the highly dopedlayers, especially juxtaposed with the highly doped GaAs layers and isepitaxied at a suitable temperature.

It will be understood that a GaAs layer as a highly doped layer and asthe intermediate layer can be applied on both boundary surfaces of theAlGaAs layer which are to be provided with the highly doped layers andepitaxied at appropriate temperatures.

The method according to the invention can be carried out advantageouslyin that during the chosen temperature of about 600° C. which is suitablefor the epitactic growth of GaAs layers, no doping atoms diffuse intothe AlGaAs layer or segregate. The AlgaAs layer is protected in thismanner from doping atoms, for example of silicon or tellurium. TheseGaAs layers provide the function of diffusion blocking or diffusionstopping layers or of segregation stopping layers for doping atoms fromadjoining highly doped regions in the layer sequence or structure. TheGaAs layer functioning as a diffusion blocking layer or segregationblocking layer ensures that the layer sequence can be formedindependently of the temperature. Consequently the layer sequence can bemade at an ideal growth temperature of about 600° C. for GaAs. This hasthe additional advantage that potential very strong heating of thecomponent even after fabrication and during its manufacture will notgive rise to any temperature dependent degradation of the component.

BRIEF DESCRIPTION OF THE DRAWING

The invention is further described based upon two embodiments and theaccompanying figures, in which:

FIG. 1 is a table describing the invention; and

FIG. 2 is two diagrams describing the invention.

SPECIFIC DESCRIPTION

FIG. 1 shows in an overview three layer sequences A-C, with no GaAslayers as diffusion or segregation blocking layers 4, 6 and an AlGaAslayer 5 as the graded layer (Layer sequence A: State of the Art), alayer sequence B with one GaAs layer as the diffusion or segregationblocking layer 4, 6, and a layer sequence C with two additional GaAslayers as diffusion-blocking or segregation blocking layers 4, 6. Thematerial for the individual layers of the layer sequence is identical inall three cases although the thicknesses can be varied as given in thecolumns A through C.

In the layer sequences A through C, the highly doped n⁺GaAs layer 1 ineach case constitutes a collector and serves as an electrical connectionto the Gunn diode.

The collector 1 as the electrical feed is a highly doped n⁺-GaAs layerdoped with 4×10¹⁸ cm⁻³ silicon. On this an n⁻-GaAs layer 2 with a lowerlevel of silicon (10¹⁶ cm⁻³ silicon) is applied as a so-called transitzone and serves to maintain the Gunn effect. Opposite to the collector1, an n_(d)-GaAs layer is arranged on layer 2. The doping of this layer3 with silicon is 4×10¹⁸ cm⁻³ for the sequence layer A and 10¹⁸ cm-3 foreach of the layer sequences B and C. Upon the 10 nanometer thick highlydoped n_(d)-GaAs-layer 3, a 50 nanometer thick AlGaAs layer 5 is appliedwhich at the boundary surface to the highly doped n_(d)-GaAs layer 3 hasan aluminum concentration of 30% which falls, over the thickness of thatlayer, steplessly to zero percent. In the case of the layer sequence C amaximum aluminum concentration of 32% is formed in the layer 5. On theAlGaAs layer 5, the highly doped n⁺-GaAs layer 7 is applied which isdoped with 4×10¹⁸ cm⁻³ silicon.

Between the N_(d)GaAs-layer 3 and the AlGaAs layer 5 in the layersequences B and C, an undoped GaAs layer 4 is additionally provided as adiffusion or segregation blocking layer. This layer is 5 nanometersthick in the case of the layer sequence B. In the layer sequence C, thislayer has a thickness of 10 nanometers. In addition on the AlGaAs layer5 in the layer sequence C a further 10 nanometer thick undoped GaAslayer 6 is provided on the AlGaAs layer.

FIG. 2 shows a comparison of the current versus voltage characteristicsof the layer sequences A through C in the forward and rearwarddirections. The current versus voltage characteristic of the layersequence A, differentiating from GaAs with an ideal growth temperatureof 600° C., shows no Schottky-typical rise in the current versus 5voltage characteristic. The curve rather begins with a straight linesignifying properties of an ohmic resistance. The curve is furthercharacterized by a symmetrical relationship for positive and negativesupply voltages.

The incorporation of a diffusion or segregation blocking layer 4 beneaththe graded AlGaAs layer 5 in layer sequence B already shows asignificant improvement in the characteristic in terms of nonlinearityand thus good functioning of the AlGaAs layer. A Schottky typecharacteristic is clear in both the forward and background directions.

Sill more pronounced is the effect when a further diffusion orsegregation blocking layer 6 (layer sequence C) is provided for thegraded AlGaAs layer 5 and a diffusion of doping atoms from highly dopedregions into the AlGaAs layer is suppressed at boundary surfaces towardhighly doped layers. Then a very pronounced Shcottky type characteristiccan be recognized (Curve C, FIG. 2).

While the characteristic for the layer sequence C demonstrates clearlythe effect of a graded emitter it can be clearly seen from the curve Athat the effectiveness of the AlGaAs layer can be lost. This lattereffect is a function of the diffusion or segregation of the silicondoping substance of layer 7 or 3 into the AlGaAs layer 5 and results inelectrical doping of the layer 5 and a change in its effectiveness.

It is conceivable within the scope of the invention to use a layersequence according to the invention for further electronic componentswhich require a potential barrier as the injection layer.

There are further layer sequences or structures conceivable in which theprinciple of protecting a graded layer, by undoped intermediate layersas diffusion-blocking or segregation blocking layers can be used, forexample in the case of InP based layer sequences. In these layersequences instead of GaAs as the material for the layer 1 to 7, InP canbe used.

1. A layer sequence or structure comprising: a first highly dopedn_(d)-GaAs layer; a graded layer of AlGaAs on the first highly dopedlayer and having an aluminum concentration that diminishes, startingfrom a boundary surface with the first highly doped layer, in thedirection of an opposite boundary surface of the AlGaAs layer; a secondhighly doped n⁺-layer; and an undoped intermediate layer juxtaposed withthe first or second highly doped layer and at least one boundary layerof the graded AlGaAs layer.
 2. A layer sequence or structure inaccordance with claim 1 wherein the undoped intermediate layer iscomposed of GaAs.
 3. A layer sequence or structure in accordance withclaim 1 wherein GaAs is the material for the second highly dopedn⁺-layer.
 4. The layer sequence according to claim 1 wherein silicon ortellurium is the doping substrate.
 5. The layer sequence or structureaccording to claim 1 wherein the layer sequence is arranged on furtherlayers.
 6. The layer sequence or structure according to claim 1 whereinthe layer sequences is disposed on a n⁻-GaAs layer.
 7. The layersequence of claim 6 wherein the n⁻-GaAs layer is disposed on a highlydoped n⁺-layer of GaAs.
 8. The layer sequence or structure according toclaim 1 wherein the first highly doped n_(d)-GaAs layer or the secondhighly doped n⁺-layer are doped with up to 10¹⁸ cm⁻³ silicon.
 9. Amethod of making a layer sequence or structure, the method comprisingthe steps of: providing a first highly doped n_(d)-GaAs layer as asubstrate having a pair of opposite boundary surfaces, forming on one ofthe boundary surfaces of the first highly doped GaAs layer an undopedGaAs layer and epitaxiing the undoped GaAs layer at an appropriatetemperature, providing on the undoped GaAs layer a graded AlGaAs layer;and providing on the other of the boundary surfaces a second undopedGaAs layer and epitaxiing the second undoped GaAs layer at anappropriate temperatures.
 10. The method according to claim 9, furthercomprising the step of providing a further highly doped GaAs layer onthe second undoped GaAs layer.
 11. A layer sequence or structurecomprising a first highly doped layer, a graded layer arranged on thefirst highly doped layer, a second highly doped layer, and an undopedintermediate layer juxtaposed between one of the highly doped layers anda boundary surface of the graded layer.